This invention relates to color electrophotography and more specifically concerns specific developers employed in a color electrophotographic process.
Color electrophotography with multiple development techniques is capable of producing color reproductions employing multiple sequencing of electrophotographic charging, exposing, and developing steps with color toners. A suitable photoconductor such as substantially panchromatic zinc oxide photoconductive paper, electrofax paper, for example, is electrostatically charged uniformly in the dark then exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photconductor. The electrostatic latent image is then developed with the complementary magenta colored toner to form a magenta-colored image corresponding to said electrostatic latent image and transferred in register to an image receiving member. A zinc oxide photoconductive paper is then again electrostatically charged uniformly in the dark and then exposed through a red filter to an imagewise projection of a color image in register with said magenta developed image to form a second electrostatic latent image which second image is developed with the complementary cyan-colored toner and likewise transferred in register. Similarly, zinc oxide photoconductive paper is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a color imaged in register with said magenta and cyan-developed images to form a third electrostatic latent image which is then developed with the complementary yellow toner and again transferred in register.
This conventional electrophotographic process with superimposed development to obtain images of cyan, magenta, and yellow, respectively, is capable of producing multicolor images by employing toners of different colors. The sequence of exposures through color filters in this multiple development process may be performed in any suitable sequence other than the green, red, and blue sequence recited above. In any event, after the desired number of exposures and developments and transfers in register, the finished image is itself fused to provide a permanent reproduction of the original.
In conventional electrophotographic imaging processes commonly referred to as xerographic imaging processes or xerography, there are numerous known carrier materials from which to select in providing a developer, i.e. a toner and a carrier combination. However, in color electrophotographic imaging the selection of appropriate carrier material is not just a matter of choice. For example, steel shot may be employed with the cyan and yellow toner materials, but steel shot is found to be unsatisfactory with the magenta toner therefore necessitating the discovery of an appropriate carrier, such as nickel berry, to perform this function. Further, the selection of a toner material of the appropriate color and triboelectric properties is in itself far more than a matter of choice. A colorant and resin combination must be selected such that the color, of course, is the appropriate hue, but more significantly that the toner possesses the appropriate triboelectric properties which will enable it to function in an electrophotographic automatic imaging mode. Few of these toner materials are known. It, therefore, becomes even a more demanding and crucial operation to develop the appropriate combination of toner and carrier so that they will develop and maintain the appropriate triboelectric relationship, this is further complicated by employing these color developers in combination in an electrophotographic imaging process.
Each developer employed comprises ordinarily a toner or a resin colorant mixture in combination with a carrier. It is a function of the toner carrier combination or developer package in a given development system, for example cascade, to assume a triboelectric relationship such that the toner will be carried with the carrier during the development cycle by electrostatic attraction and then selectively deposited charge-wise on the electrostatic latent image which has a greater affinity for the toner electrostatically than does the carrier particle. In addition to the very significant triboelectric properties that a developer must possess and maintain during the development cycle, the toner must possess the appropriate color and continue to function under machine conditions which expose the developer to impaction and humidity among other undesirable factors. It is, therefore, critical that a specific toner which comprises a colorant and a resin be combined with a specific carrier of the appropriate size relation to the toner particle so that the appropriate color is exhibited and the appropriate triboelectric relationship is maintained to achieve successful development. Under continuous electrophotographic imaging, it has been found that with regard to one developer package great difficulty is realized in maintaining the proper triboelectric relationship so that proper development is achieved. With the advent of a three-color system, the complexities of producing suitable cyan, magenta, and yellow developers which will cooperate in a color electrophotographic continuous imaging process, as above described, are further exaggerated. It is apparent that any one of a number of variables in any one of the developers could cause incomplete, improper, or inadequate developments so that the color balance is thereby shifted resulting in a completely unacceptable color print.
There are also uses for colored toners in applications where full color is not needed but it is desirable that copies can be made other than in black. These single colored copies may be useful to indicate security classification or to draw attention to the communication as being of particular importance.
A yellow toner which has been found particularly suitable for formation of yellow images in both sequential full color copiers and copiers less than full color reproduction is the toner comprising a yellow colorant and a resin material, said colorant comprising a compound the formation of which is generally described in U.S. Pat. No. 2,644,814, the compound satisfying the formula: ##STR1## This pigment is classified in the Colour Index as Pigment Yellow 97. Toners containing this colorant are hereinafter referred to as Yellow 97 toners.
The Yellow 97 toners have been employed with a methyl terpolymer coated carrier to form a developer. The methyl terpolymer carrier has a coating of styrene/methyl methacrylate/vinyltriethoxysilane terpolymer. While generally successful in certain instances in automatic imaging processes, problems arise in connection with the useful life of these developers. It is believed that these problems are associated with high static charge generated in the developer sump. The charge is of the same polarity as applied to the magnetic brush employed in the development system to suppress background. The image charge is found to be above the background bias and, for some not fully understood reason, because of the static charge on the coated carrier, the background bias is elevated and prevents development of the image.